The present invention provides a ferritic-austenitic stainless steel provided for seawater applications and use of this ferritic-austenitic stainless steel in seawater applications and nearby areas, where especially favorable properties for the steel have been attained.
Ferritic-austenitic (duplex) stainless steels are widely used today as construction material in a number of industries. Duplex steels are often developed for especially favorable use in special areas. For example, the duplex steel SAF 2507 (UNS S 32750), which is alloyed with 25% Cr, 7% Ni, 4% Mo and 0.3% N and which is described in the Swedish Patent Application SE-A-453 838, concerned to be especially resistant against chloric induced corrosion and finds therefore applications as construction material if the process solution contains chlorides or if the material will be exposed for seawater or chlorine containing cooling water, for example in heat exchangers.
In U.S. Pat. No. 5 582 656 (SE-A-501 321) duplex steels are described, which contain a maximum of 0.05 weight % C, a maximum of 0.8 weight % Si, 0.3-4 weight % Mn, 28-35 weight % Cr, 3-10 (3-7) weight % Ni, 1.0-3.0 (1.0-4.0) weight % Mo, 0.30-0.55 weight % N, a maximum of 1.0 weight % Cu, a maximum of 2.0 weight % W, 0.010 weight % S and 0.2 weight % Ce, and a balance of Fe together with normally occurring impurities and additives, and wherein the ferrite content of the steel makes 30-70 volume %.
A purpose of the present invention is to provide duplex steel for use within seawater applications.
As described in SE-A-453 838 the composition of the alloy is not the most important factor to provide such steel. The balance between the different components of the alloy and structural factors is more important. Furthermore it is well-known from this patent that high amounts of, for example, chromium, improve the tendency of precipitation of intermetallic compounds so strong, that problems in manufacturing and in relation with welding could occur. A high amount of nitrogen is desired in order to stabilize the alloy against precipitation of intermetallic phases and improvement of the corrosion resistance, but is restricted by the limited solubility in the melt, which causes precipitation of chromium nitrides. By these reasons the content of chromium in this alloy will be restricted to a maximum of 7% and the content of nitrogen to 0.25-0.40%.
Surprisingly, some of the alloys described in U.S. Pat. No. 5,582,656 have been found to possess favorable and, in certain cases, particularly good properties as construction material in the field of seawater applications. This result is surprising since these alloys have a high content of chromium and high content of nitrogen that is over the upper limit that taught by to SE-A-453 838 as avoiding precipitation. Especially good properties will be achieved if the PRE-value of the steel is at least 40.
The invention provides consequently to a steel containing a maximum of 0.05 weight % C, a maximum of 0.8 weight % Si, 0.3-4 weight % Mn, 28-35 weight % Cr, 3-10 weight % Ni, 1.0-4.0 weight % Mo, 0.2-0.6 weight % N, a maximum of 1.0 weight % Cu, a maximum 2.0 weight % W, a maximum of 0.010 weight % S and a maximum of 0.2 weight % Ce, and the balance Fe together with normally occurring impurities and additives, at which the ferritic content makes 30-70 volume % and the PRE-value is at least 40.
None of the steel grades, that are specifically described in U.S. Pat. No. 5,582,656 or SE-A-501 321 provides a PRE-value over 40 in both the ferritic and the austenitic phase. Most of the embodiments provide a PRE-value under 40 even calculated on the hole composition.